NONINVASIVE IN VIVO IMAGING OF CD4 CELLS IN SHIV INFECTED NONHUMAN PRIMATES The number of CD4+ T cells in the peripheral blood is recognized as a major prognostic determinant of AIDS in HIV 1 infected individuals. However, because only 2% of total body lymphocytes circulate in the blood, CD4+ T cell determinations from this source are likely not to be representative of total body levels of this T cell subset. Consequently, total body imaging, employing single-photon emission computed tomography (SPECT) scanning of uninfected and SHIV infected macaques in conjunction with the gamma emitter indium-111, was used to study the CD4+ T cell pool in vivo. A strong correlation was observed between radiotracer uptake in spleen, tonsil, axillary lymph nodes, and peripheral blood CD4+ T cell levels (&#961;= 0.75, 0.93, and 0.85, respectively, P <.005). These data also provided a new estimate for the total number of lymphocytes in the body (based on a calculation of 1.1 x 10e9 CD4+ T cells/gram of spleen tissue) as between 1.9 and 2.9 x 10e12 cells and would suggest that the partition between peripheral blood and lymphoid tissue is between 0.3 and 0.5%. GENERATION OF AN AID-INDUCING R5-TROPIC SHIV WITH NOVEL PATHOGENIC PROPERTIES BY SERIAL PASSAGING IN RHESUS MACAQUES We have generated a new pathogenic R5-tropic SHIV following long-term animal-to-animal passage, which bears the env gene from the prototypical macrophage tropic strain, HIV 1Ada. SHIVAD8 is able to sustain plasma viremia in rhesus monkeys for more than 2 years;all 13 inoculated animals experienced marked depletions of CD4+ T lymphocytes. Ten of these monkeys became normal progressors (NPs), generated anti-viral CD8+ T cell responses, and sustained chronic immune activation. Several of these animals also produced high titers of NAbs during the course of their infections. The most consistent and distinguishing in vivo property of the SHIVAD8 family of viruses is the slow and unremitting loss of both memory and nave CD4+ T cells. This pattern of depletion was observed in all 10 NPs. During the chronic phase of infection, the loss of nave CD4+ T cells was more rapid and marked than the depletion of the memory subset. By week 80, NPs had sustained an 87 to 93% loss of nave cells from their pre-inoculation levels. An assessment of coreceptor utilization of late stage viruses recovered from SHIVAD8 NPs indicated that a coreceptor switch had not occurred and the virus present throughout the infection had remained R5 tropic. Surprisingly, and in contrast to both SIVmac and SIVsmE lineages, the pace of CD4+ T lymphocyte decline did not correlate with plasma virus loads. Although the geometric mean plasma viral RNA level, at week 50 in the SHIVAD8 infected monkey cohort was 1.7 x 10e3 RNA copies/ml, the set-point virus loads varied widely in different animals (1.6 x 10e2 to 1.5 x 10e5 RNA copies/ml). Thus far, 3 of the 10 SHIVAD8 NPs have developed clinical symptoms of immunodeficiency requiring euthanasia between weeks 100 and 117 PI. These monkeys sustained opportunistic infections involving Mycobacterium avium, Pneumocystis carinii, and Campylobacter coli. In addition, two other NPs have lost a significant amount of weight, are experiencing chronic diarrhea, and currently have total circulating CD4+ T cell counts of 39 and 75 cells/&#956;l, respectively.